Phenolic composition, sugar contents and antioxidant activity of Tunisian sweet olive cultivar with regard to fruit ripening.

Dhokar olive cultivar growing in the south of Tunisia is an unusual olive tree which is characterized by the sweet taste of its fruit. We were particularly interested in the phenolic compounds of this cultivar in comparison to Chemlali cv. During the different maturation stages of olive fruits, the phenolic composition, sugar content, antioxidant activity and enzymatic change were examined. The phenolic composition was studied by using reverse-phase high-performance liquid chromatography followed by LC-MS and GC-MS analysis. Oleuropein, the major olive fruit biophenolic compound, decreased significantly during all the ripeness stages, and its level decreased from 3.29 g/kg fresh olive (July) to 0.16 g/kg (October) in Dhokar cv. and from 5.7 g/kg (July) to 3.75 g/kg (October) in Chemlali cv. This decrease inversely correlated with hydroxytyrosol concentrations until September. DPPH and ABTS assays show that the more important antioxidant capacity of olive extracts was found at the last stage of maturation. beta-Glucosidase and esterase activities were evaluated during the ripening stage. The data obtained during the ripening indicate that polyphenol content and composition, in particular the oleuropein concentration, were in correlation with the measured enzymatic activities. Glucosidase and esterase showed their maximum values in September reaching 179.75 and 39.03 U/g of olive pulp, respectively. In addition, sugar content was quantified using colorimetric and chromatographic methods and was compared to its concentration in Chemlali olive fruit cultivar. Glucose and mannitol were the main sugars; they reached their highest level at the last stage of ripening: 8.3 and 79.8 g/kg respectively.